Acquired Immunodeficiency Syndrome (AIDS) and cancer are major world health problems today. Despite recent advances in anti- retroviral therapy, it is clear that current measures are insufficient to eradicate AIDS. In order to develop better anti- retroviral strategies, a more complete understanding of the biology of retroviruses is essential. This proposal outlines a novel approach designed to understand the process of retroviral replication and to elucidate the roles played by the different viral proteins in the various steps of infection. A detailed knowledge of these processes is also important for understanding basic biological processes such as cellular growth control, and for advances in biotechnology such as in creating better retroviral vectors for gene therapy. Aim 1 consists of creating several comprehensive libraries of mutants in a retroviral sequence using the new technique of genetic footprinting. Genetic footprinting is an efficient method for construction and parallel analysis of a very large set of mutations in a sequence. Aim 2 consists of identifying regions of the retroviral genome that are essential for viral replication. Since genetic footprinting allows the identification of functionally essential regions in a gene without isolating or sequencing individual mutants, it will be applied to retroviral sequences, starting with gag, the region coding for structural proteins that form the viral capsid. This will define all regions in gag that are essential for viral replication. Aim 3 consists of identifying regions in the retroviral genome that are essential for individual steps of viral entry and replication. Aim 4 consists of performing a detailed analysis of some of the mutants identified in the above screens using standard biochemical and cell-biological methods of studying viral entry and replication. This will help identify the specific roles of different regions of a gene in greater detail. The experiments are designed to provide extensive data on the process of retroviral replication and infection.